The most commonly applied approach to field emission incorporates an atomically sharp tip that concentrates electric field lines of an applied potential, thus effecting local geometric field enhancement at the cathode-vacuum interface. The enhanced field facilitates electrons' tunneling into vacuum through the steep energetic barrier at the emission surface. Arrays of geometric field enhancement cathodes generally require an applied potential of 20 to 200 V to obtain practical working current densities, i.e. greater than 1 mA/cm.sup.2.
Another approach employs as the cathode material an impurity-doped semiconductor having a surface with a negative electron affinity ("NEA"). The conduction band edge of such a material is higher than the vacuum energy level, so emission of conduction band electrons to vacuum from the bulk of the cathode body is energetically favored. However, in general there is a significant barrier to injection of electrons into the cathode material from a metal contact. If the semiconductor forms a Schottky diode on metals, then when an electric field is applied across the semiconductor, the dopant impurities become positively ionized and form a depletion region at the metal-semiconductor junction, giving rise to a local field enhancement. When nitrogen-doped diamond is used as this semiconductor, the electric field at this junction is often greater than 10.sup.7 V/cm, sufficient to cause tunneling of electrons from the metal into the semiconductor. Once the electrons are in the NEA semiconductor they can be emitted directly into vacuum. The dopant-induced field enhancement at the metal-semiconductor interface may be augmented by interface morphology, as described in U.S. Pat. No. 5,713,775 (FIELD EMITTERS OF WIDE BANDGAP MATERIALS AND METHODS FOR THEIR FABRICATION, filed May 2, 1995), hereby incorporated by reference. Cathode arrays using a dopant-induced electric field enhancement technique typically require 10 to 20 V to obtain practical working current densities.
Although excellent electron emission characteristics have been observed from known emitter structures, especially those incorporating cathode bodies of diamond and amorphous diamond-like materials, practical applications of these cathodes remains limited by a lack of performance reproducibility. Also, the electrons enter the vacuum from thermal equilibrium with the cathode body bulk, where they have been subject to many collisions; thus they have little kinetic energy and require an accelerating voltage in the region vacuum adjacent the emitting surface. Finally, field emitters operable using smaller working voltages would lower power requirements and make field emitters suitable for incorporation into a wider range of devices.